There are a number of commercial applications for controlled gas atmosphere enclosures. For example, electrical components and circuits are often tested in enclosures at a selected temperature and/or relative humidity for a period of time. Another common application for controlled atmosphere enclosures is the growth of biological cultures in the laboratory. As will be discussed herein with regard to a particular embodiment, the present invention may be advantageously employed in connection with a controlled gas atmosphere incubator in which a chamber for biological cultures is heated and in which the atmosphere of the chamber is controlled as to one or more constituent gases and/or the relative humidity.
A typical incubator of the foregoing type includes a generally cubical outer housing made up of five insulated walls (top, bottom, left side, right side, and rear) and an insulated front door. The door is mounted on hinges on the front of one of the side walls and may be opened to permit access to the interior of the incubator. When the door is closed, it is suitably sealed about its periphery to the housing walls to form the sixth wall of the housing. The incubator chamber, in which biological cultures are grown, is formed by inner walls, inside the insulated outer walls, and typically includes shelves upon which culture containers are placed. The shelves are carried by suitable shelf supports inside the chamber.
Most incubators of this type are either water jacket incubators or forced draft incubators. In a water jacket incubator the inner chamber is heated to the desired temperature by a sealed jacket of water surrounding the five fixed sides of the incubator chamber. The water jacket lies between the chamber wall and the insulated housing walls and is heated by heating elements in thermal contact with the water in the water jacket. Due to the thermal conductivity of water, the heat from the individual heating elements is relatively evenly dispersed over the water in the water jacket, providing even heating of the chamber. Such even heating is desirable in order to provide a uniform temperature for the biological cultures in different areas within the chamber and in order to prevent “cold spots” on the inner chamber wall upon which condensation can form.
Although the heating of the chamber walls in a water jacket incubator is substantially uniform, the chamber atmosphere will stratify thermally if the chamber atmosphere is undisturbed. When such stratification occurs, the temperature of the chamber atmosphere is greater at the top of the chamber than at the bottom of the chamber. In addition, if a constituent gas concentration is maintained in the chamber, such as a particular CO2 level, the constituent gas will also stratify within the chamber atmosphere. Consequently, it is desirable to maintain a certain rate of flow within the chamber to assure uniformity of temperature and of constituent gases. In order to do this, typically a portion of the chamber is separated from the main chamber area by a wall to define a duct extending, for example, along a side of the chamber. A small blower or fan is placed in the duct and the chamber atmosphere is circulated, such as from a duct inlet in the upper portion of the chamber to a duct outlet in a lower portion of the chamber.
In a forced draft incubator, the inner chamber walls are insulated from the outer housing walls by a layer of insulation inside the housing walls. However, in this case there is no water jacket interposed between the insulated outer walls and the inner chamber walls. To obtain heating of the chamber in a forced draft incubator, some type of duct, such as described above, is typically provided within the chamber, and a fan and a heating element are mounted in the duct. As the fan circulates air from the main chamber area through the duct, the circulated chamber atmosphere is heated by the heating element. In order to heat the chamber atmosphere substantially uniformly, and to the desired temperature, considerably greater air flow is required than in the case of a water jacket incubator.
In a typical forced draft incubator, or water jacket incubator, if a constituent gas in the atmosphere of the incubator chamber is to be maintained at a particular level, a probe is introduced into the chamber, perhaps within the duct through which the chamber atmosphere circulates. In the case of CO2, for example, a CO2 sensor is introduced into the incubator chamber to measure the concentration of CO2 therein. A source of CO2 is then coupled to the interior of the chamber through a controlled valve, with an automatic control system actuating the valve as required to maintain the CO2 concentration in the chamber at a selected level.
The humidity in a forced draft incubator is also often controlled. Rather than introducing steam or water into the incubator chamber as may be done in the case of a water jacket incubator, in a forced draft incubator quite often a pan of water is placed upon the floor of the incubator chamber, and the recirculated chamber atmosphere is directed out of the bottom of a duct across the surface of the water in the pan. Due to the higher recirculation rates in a forced draft incubator, appropriate humidification of the chamber is obtained.
In either a forced draft or a water jacket incubator, sensors such as for CO2 or humidity have typically been located within the chamber atmosphere itself, although perhaps within a recirculation duct, as earlier described. Such sensors in the chamber are subject to the chamber atmosphere, and a sensor can fail or suffer performance degradations due to contaminants or the accumulation of a coating on the sensor. The presence of such sensors in the incubator chamber itself also makes cleaning of the chamber interior more difficult. In fact, the very existence of a duct or the like for the circulation of the chamber atmosphere within the chamber introduces difficulties in cleaning the chamber.
The recirculation of the chamber atmosphere, such as through a duct, in either type of incubator presents yet another problem, that of potential contamination of biological cultures within the chamber. Contaminants such as mold spores are almost invariably present in the chamber atmosphere and may be directed by the recirculatory air flow into the biological culture containers. Culture contamination problems are exacerbated by the higher air flows required in forced draft incubators.
Higher air flow rates involved in forced draft incubators have a further disadvantage in that the higher flow rates tend to dry out biological culture media. To a large degree, the necessity of offsetting this desiccation results in the requirement for humidity control in forced draft incubators. In such incubators, a relatively high humidity is maintained so that the drying effect of the gas flow is ameliorated.
Incubators are typically used for growing cultures in a controlled environment wherein both temperature and atmospheric gas concentration are maintained at selected levels. For certain applications it is highly desirable to have both temperature and gas concentrations maintained within strict tolerances while still allowing easy access to the incubator chamber for adding or removing items to and from the chamber or for inspecting the contents of the chamber. Control of environmental variables is desirable to maintain accuracy and reproducability of incubation results.
Therefore, it would be desirable to provide an incubator having accurate gas concentration control with fast recovery of gases (typically CO2 combined with O2 or N2) by determination of the total gas volume required including gas loss due to same gas injection and gas loss due to other gas injection.